Caption: Plant meiosis. Light micrograph of a transverse section through the anther of a lily (Lilium sp. ), showing pollen grains undergoing meiosis. Part of a pollen sac is seen, containing rounded pollen grains. These pollen grains are undergoing meiosis (reduction division), whereby normal diploid cells divide by two divisions to form four haploid cells (gametes). Chromosomes are black structures within each pollen grain. Some chromosomes show chiasmata, genetic crossing-over, where the arms of homologous chromosomes join and fuse to enable genetic recombination to occur. See cell at upper centre left: a chromosome resembling an X appears to have one long arm.